Friday, July 11, 2008

Hardness

One of the best ways to think of hardness is as the scratch-resisting ability of the gem. Hardness is directly related to the tenacity of atomic attractions. There are great differences in the strengths of the bonds by which different kinds of atoms are held together. Naturally, some combinations will resist being torn apart more than others. The atom bonding in diamond jewelry is so very strong that the species is exceptionally hard and cannot be scratched or torn apart by other substances. Softer gems, such as amethyst, are much less strongly bonded and are soft enough so that repeated exposure to scratching forces will leave them badly marked. Since the early 1800's a rough but convenient scale for measuring hardness, originated by the German mineralogist Friedrich Mohs, has been in general use. The scale is based on ten relatively common minerals ranked from 1 to 10 in the order of increasing hardness: 1) talc, 2) gypsum, 3) calcite, 4) fluorite, 5) apatite, 6) feldspar, 7) quartz, 8) topaz, 9) corundum, 10) diamond. The degree to which hardness increases between the numbers is not at all uniform. There is a greater difference between the hardness of corundum and diamond—9 and 10 in the scale—than between numbers 1 and 9. Almost all important gemstones have a hardness above 6 in this scale. Anything less than 6 is not durable enough to resist the scratching and chipping of general use. For practical purposes it is useful to know that window glass is usually slightly softer than 6, a good steel knife is 6 to 61/2, and a hard file is close to 7. One of the odd variations in the hardness of some gemstones occurs when there is an appreciable difference in the strengths of the atomic bonds in different directions through the structure. Kyanite, for example, varies from 5 to 7 in hardness, depending on the direction in which an attempt is made to scratch it.

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